You cannot add any custom headers this way.
The established way to add user specific authorization is to have a session cookie. If such a cookie is set for the target URL (i.e. the user is already authenticated against the target) then it will be sent automatically within the request.
Yet another way is if the target sends a "authentication required" (HTTP status code 401) request back with an appropriate
WWW-Authenticate header. In this case the browser will prompt the user for the login credentials and send these with each request to the site using the